Highly soluble carbon nanotubes with enhanced conductivity

ABSTRACT

New methods for preparing carbon nanotube films having enhanced properties are provided. The method broadly provides reacting carbon nanotubes (CNTs) and compounds comprising a polyaromatic moieties in the presence a strong acid. During the reaction process, the polyaromatic moieties noncovalently bond with the carbon nanotubes. Additionally, the functionalizing moieties are further functionalized by the strong acid. This dual functionalization allows the CNTs to be dispersed at concentrations greater than 0.5 g/L in solution without damaging their desirable electronic and physical properties. The resulting solutions are stable on the shelf for months without observable bundling, and can be incorporated into solutions for printing conductive traces by a variety of means, including inkjet, screen, flexographic, gravure printing, or spin and spray coating.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/530,471, entitled HIGHLY SOLUBLE CARBON NANOTUBES WITH ENHANCEDCONDUCTIVITY, filed Jun. 22, 2012, which claims the priority benefit ofU.S. Provisional Patent Application No. 61/500,985, filed Jun. 24, 2011,entitled THE USE OF STRONG NON-OXIDATIVE ACIDS AND PYRENE DERIVATIVES TOPRODUCE HIGHLY SOLUBLE CARBON NANOTUBES WITH ENHANCED CONDUCTIVITY, eachof which is incorporated by reference herein in their entirety.

FEDERALLY SPONSORED RESEARCH/DEVELOPMENT PROGRAM

This invention was made with government support under contract number70NANB10H001 awarded by The National Institute of Standards andTechnology's Technology Innovation Program. The United States Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned with noncovalently functionalizingcarbon nanotubes using strong acids and functionalized polyaromaticmolecules in order to increase their solubilities and/or conductivities.

2. Description of the Prior Art

Carbon nanotubes (CNTs) have shown great promise for conductive traceapplications, especially in printed electronics. Printed CNT tracesoffer a number of benefits over traditional metal traces, including easeof application and of mechanical flexibility. However, “raw” CNTs areusually produced in a very disordered and impure powder, and must bepurified and dispersed to create the conductive (or semiconductive)“inks” used to print the CNT traces. Getting CNTs to remain dispersed insolution, however, can be a challenge. CNTs very strongly attract eachother due to van der Waals forces, causing them to agglomerate and fallout of solution. In order to create useful CNT inks for printing,processes must be developed to ensure that the CNTs remain dispersed.

Several methods have been used to make carbon nanotubes (CNTs) moredispersible, including oxidation processes, the use of surfactants,covalent functionalization with solubilizing groups, and non-covalentfunctionalization. Of these methods, non-covalent functionalization hasthe least effect on the electronic properties of the carbon nanotubes.Covalent functionalization creates defects in the pi network of theCNTs, which adversely affects their conductivity. Similarly, oxidationof the nanotubes will negatively affect the electronic characteristicsof the CNTs, as the oxidation damages the tubes and could decrease theirsize. The addition of additives to the solution, such as surfactants,can also disrupt the electronic properties of the final ink-printed CNTfilms. To reduce this effect, post-applications treatments, includingrepeated washings, of the printed CNT films are necessary, which createsextra steps, yield lost, and large amounts of waste, and still may notrestore the conductivity of the original CNTs.

Several methods of non-covalently functionalizing carbon nanotubes havebeen published. If the functional group is a liquid, then simplystirring at a raised temperature can be effective. Some solids can bemelted with the carbon nanotubes but many solids decompose beforemelting, which is particularly the case for many polyaromatichydrocarbons. Sonication can also be used to temporarily disperse CNTsin a solvent. Sonication can be used in conjunction with anotherfunctionalizing method, since it temporarily breaks up the carbonnanotube bundles and allows the functionalizing groups to get betweenthe CNTs. However, strong or prolonged sonication has a tendency todamage carbon nanotubes, which likely results in less than desirableelectronic properties.

There is a need for improved methods of solubilizing carbon nanotubeswhile preserving, and even enhancing, their conductivity.

SUMMARY OF THE INVENTION

The present invention overcomes the problems of the prior art byproviding a method of preparing a carbon nanotube dispersion withimproved carbon nanotube solubilities and conductivities. The methodcomprises providing a mixture of carbon nanotubes, a compound comprisingat least one polyaromatic moiety, and an acid. The compound comprisingat least one polyaromatic moiety is noncovalently bonded with the carbonnanotubes, and the acid is reacted with the at least one polyaromaticmoiety. The invention is also directed towards the dispersion formed bythis method.

In another embodiment, the invention provides a dispersion comprisingcarbon nanotubes noncovalently bonded to compounds comprising respectivepolyaromatic moieties. At least some of the polyaromatic moieties arereacted with an acid. The dispersion has a carbon nanotube concentrationof at least about 0.05% by weight, based upon the total weight of thedispersion taken as 100% by weight. The dispersion is also formable intoa film having a sheet resistance of less than about 7,000 SI/sq.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the D/G ratio of the sample prepared inExample 1;

FIG. 2 is a graph showing the TGA analysis of the sample prepared inExample 1;

FIG. 3 is a graph of the TGA analysis of raw CG200 carbon nanotubes;

FIG. 4 is a graph depicting the D/G ratio of the sample prepared inExample 2;

FIG. 5 is a graph showing the D/G ratio of raw CG200 carbon nanotubes;

FIG. 6 is a graph illustrating the Raman spectrum of the sample preparedin Example 2;

FIG. 7 is a graph depicting the Raman spectrum of raw CG200 carbonnanotubes;

FIG. 8 is a graph depicting the D/G ratio of the sample prepared inExample 3;

FIG. 9 is a graph showing the D/G ratio of raw SG65 carbon nanotubes;

FIG. 10 is a graph illustrating the Raman spectrum of the sampleprepared in Example 3;

FIG. 11 is a graph depicting the D/G ratio of the sample prepared inExample 4;

FIG. 12 is a graph illustrating the Raman spectrum of the sampleprepared in Example 4;

FIG. 13 is a graph depicting the D/G ratios of the sample prepared inExample 5 and of XBC3350 carbon nanotubes (for readability, the spectrumfor the XBC3350 was multiplied by a factor of 0.473);

FIG. 14 is a graph depicting the Raman spectra of the sample prepared inExample 5 and of XBC3350 carbon nanotubes (for readability, the spectrumfor the XBC3350 was multiplied by a factor of 0.473);

FIG. 15 is a graph showing the D/G ratios of the second sample preparedin Example 6 and of CG200 carbon nanotubes (for readability, thespectrum for the CG200 was multiplied by a factor of 4.212);

FIG. 16 is a graph depicting the Raman spectra of the second sampleprepared in Example 6 and of CG200 carbon nanotubes (for readability,the spectrum for the CG200 was multiplied by a factor of 4.212);

FIG. 17 provides the NMR spectrum of the reaction product of1-pyrenemethylamine hydrochloride with fuming sulfuric acid;

FIG. 18 shows the mass spectrum of the reaction product of1-pyrenemethylamine hydrochloride with fuming sulfuric acid; and

FIG. 19 shows the chemical structure of the reaction product of1-pyrenemethylamine hydrochloride with fuming sulfuric acid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Inventive Method

The inventive method is broadly directed towards reacting CNTs,polyaromatic moieties (provided as part of a compound comprising atleast one polyaromatic moiety), and an acid. The CNTs can first be addedto the acid for breaking apart the CNTs that may be bundled or clusteredtogether, followed by addition of the compound comprising at least onepolyaromatic moiety.

Alternatively, the compound comprising at least one polyaromatic moietycan be combined with the acid, followed by addition of the CNTs. As afurther alternative, the compound comprising at least one polyaromaticmoiety and CNTs could first be combined, followed by addition of theacid.

Regardless of the order of addition, the mixture should be stirred untila consistent dispersion is achieved, i.e., until the dispersion isunchanging and essentially stable. This will typically take from about 6hours to about 4 days, more preferably from about 10 hours to about 2days, and even more preferably from about 12 hours to about 24 hours.The temperature of the mixture during this time is preferably from about0° C. to about 100° C., more preferably from about 15° C. to about 60°C., and even more preferably from about 20° C. to about 25° C.

During this reaction process, several things occur, typicallysimultaneously. The acid works to break apart any CNT clusters orbundles. The polyaromatic moieties will (reversibly) associate with theCNTs. Preferably, the polyaromatic moieties non-covalently bond with theCNTs. As used herein, the term “non-covalent bonding” is used to referto bonding that does not involve the intimate sharing of pairs ofelectrons, as in covalent bonding, but rather involves more dispersedvariations of electromagnetic interactions. Preferred examples ofnon-covalent bonding include hydrogen bonding and electrostatic,intermolecular attraction.

Additionally, the acid preferably functionalizes the compound comprisingat least one polyaromatic moiety (e.g., the acid might sulfonate thecompound comprising at least one polyaromatic moiety). Advantageously,this process can be accomplished without sonication. It is alsopreferred that the CNTs are not oxidized during this reaction and alsoremain unfunctionalized (or are at least not further functionalizedbeyond possible sulfonation that could occur at established defectsites).

After a consistent reaction mixture has been achieved, the temperatureof the mixture is preferably lowered to a range of from about −5° C. toabout 40° C., and even more preferably from about −5° C. to about 10° C.This can be accomplished by transferring the resulting dispersion to iceor ice cold water. At this stage, the pH of the dispersion is from about0 to about 1, and more preferably from about 0 to about 0.5. The pH ispreferably adjusted by addition of a base (e.g., concentrated ammoniumhydroxide) to the highly acidic dispersion, raising the pH to a range offrom about 0 to about 10, and even more preferably from about 0 to about8. The solution is then preferably filtered and washed with deionized(“DI”) water and dilute ammonium hydroxide to yield the inventive carbonnanotube solid or dispersion (depending upon whether cross flowfiltration was used) that can be used to prepare inks, for example.

Ingredients for Carrying Out Inventive Method

Suitable CNTs for use in the present inventive method include any rawsingle-walled, double-walled, or multi-walled CNTs (SWCNTs, DWCNTs, andMWCNTs, respectively). Preferably, the CNTs are pristine, that is, CNTshaving little or no sidewall defects, existing functionalization, ordoping. Non-pristine CNTs may be used, but the existingfunctionalization or doping may be damaged by the acid treatment, andthe resulting conductivity might be affected. Exemplary types of CNTsfor this process include, but are not limited to, CG200 CNTs and SG65CNTs (available from SWeNT), XBC3350 CNTs (available from CCNI), HiPco™CNTs (available from Nanolntegris), as well as those available fromThomas Swan and CheapTubes.

Suitable compounds comprising at least one polyaromatic moiety for usein the inventive method include any unsubstituted or substitutedpolyaromatics that possess a physical and electronic structure allowingthem to be non-covalently bonded to the surface of the CNTs. Preferably,the polyaromatic moieties are planar or have a large planar area andcontain carbon ranges from about C₁₀ to about C₁₀₀, more preferably fromabout C₁₂ to about C₃₀, and even more preferably from about C₁₆ to aboutC₂₀. Exemplary polyaromatic compounds include substituted (at anyposition) and unsubstituted versions of compounds selected from thegroup consisting of naphthalene, anthracene, phenanthracene, pyrene,tetracene, tetraphene, chrysene, triphenylene, pentacene, pentaphene,perylene, benzo[a]pyrene, coronene, antanthrene, corannulene, ovalene,graphene, fullerene, cycloparaphenylene, polyparaphenylene, cyclophene,and similar molecules, as well as compounds containing moieties of theforegoing. Exemplary substituted polyaromatic compounds include thoseselected from the group consisting of 1-pyrenebutyric acid,1-pyrenemethylamine hydrocholoride, rubrene, pyrene, and triphenylene.

Suitable acids for use in the inventive process include any strong acid(and preferably a sulfonating strong acid) or superacid. Preferably theacid has a pKa of less than about −1, preferably less than about −12,and more preferably from about −12 to about −14. Exemplary acidsinclude, but are not limited to, sulfuric acid (oleum), chlorosulfonicacid, triflic acid, p-toluenesulfonic acid, and mixtures thereof.

The CNTs and compound comprising at least one polyaromatic moiety arepreferably utilized in the inventive method at levels such that themolar ratio of CNTs to polyaromatic moieties is from about 25:75 toabout 75:25, preferably from about 35:65 to about 65:35, more preferablyfrom about 45:55 to about 55:45, and even more preferably about 50:50.The acid (or acids, if a mixture of acids is utilized) is preferablyutilized at levels of from about 90% to about 99.99% by weight, morepreferably from about 95% to about 99.9% by weight, and even morepreferably from about 98% to about 99.8% by weight, based upon the totalweight of the dispersion taken as 100% by weight.

In one embodiment, the resulting dispersion is essentially free ofsurfactants. That is, surfactants are utilized in the method and/orincluded in the final dispersion at levels of less than about 1% byweight, preferably less than about 0.5% by weight, and more preferablyabout 0% by weight, based upon the total weight of the CNTs taken as100% by weight.

In another embodiment, the CNT dispersions consist essentially of, oreven consist of, the CNTs, compound comprising at least one polyaromaticmoiety, and acid (where at least some and maybe all of the acid isreacted with the compound comprising at least one polyaromatic moiety).

Resulting Dispersions and Uses Thereof

It will be appreciated that the above-described dual functionalizationof the inventive method allows the CNTs to be dispersed atconcentrations of greater than about 0.5 g/L (about 0.05% by weight),preferably greater than about 1 g/L (about 0.10%), preferably greaterthan about 1.5 g/L, and more preferably from about 1.5 g/L to about 3g/L without damaging their desirable electronic properties. Furthermore,further post-processing steps are not needed beyond addition of anysolvents for further dispersing the CNTs, preparing inks, etc. That is,conductive additives or dopants are not needed once the CNTs aredispersed.

Because damage to the CNTs is minimized, or even avoided, during thisprocess, the D/G ratio of the CNTs in the resulting dispersion is withinabout 0.2, preferably within about 0.1, and more preferably within about0.05 of the D/G ratio of the raw CNTs used to prepare the inventivedispersion.

The resulting solutions or dispersions are shelf-stable (i.e., noobservable bundling) for at least about 2 weeks, preferably at leastabout 3 months, and more preferably at least about 6 months.Furthermore, because the CNT functionalization is non-covalent, it doesnot disrupt the it network, but instead leaves the electronic structureintact so that films made from the resulting CNTs are highly conductive.The functionalization of the compound comprising at least onepolyaromatic moiety also serves to increase the conductivity of filmsformed from the resulting CNT dispersion. This functionalizationincreases the solubility of the compound comprising at least onepolyaromatic moiety and provides π-π interactions between thefunctionalized polyaromatic hydrocarbons and the CNTs.

The dispersions can be used to formulate inks to print highly conductivetraces for printed electronic applications. Films can be formed from thedispersions or inks using known methods (e.g., including as inkjet,screen, flexographic, gravure printing, or spin and spray coating). Theresulting films have high conductivities and low sheet resistances. Moreparticularly, the sheet resistance will be less than about 7,000 Ω/sq at85% T, preferably less than about 2,000 Ω/sq at 85% T, and morepreferably from about 300 Ω/sq to about 600Ω at 85% T. Additionally,with the inventive method, there is no need for a wash step after thecoating has been deposited. The above properties allow the inventivedispersions and films to be useful in numerous electronic devices,including interconnects, integrated circuits, microelectronics,optoelectronics, photoelectronics, microelectromechanical systems(MEMS), photovoltaics, sensors, and LEDs.

EXAMPLES

The following examples set forth preferred methods in accordance withthe invention.

It is to be understood, however, that these examples are provided by wayof illustration and nothing therein should be taken as a limitation uponthe overall scope of the invention.

Example 1 Reaction of Carbon Nanotubes with 1-Pyrenebutyric Acid

In this procedure, 203 mg of CG200 carbon nanotubes (Product No. 724777,SouthWest NanoTechnologies, Norman, Okla.) and 205 mg of 1-pyrenebutyricacid (Product No. 257354, Sigma Aldrich, St. Louis, Mo.) were placed ina 250-ml Schlenk flask and flushed with nitrogen. Then, 60 mL of fumingsulfuric acid (20% free SO₃, Product No. 435597, Sigma Aldrich, St.Louis, Mo.) were cannulated into the flask. The solution was stirredovernight at room temperature. An attempt to quench the solution intocold water was made, but the cannula clogged. Fuming sulfuric acid wascannulated into the flask until the solution was free flowing, and thesolution was again stirred overnight at room temperature. The solutionwas free-flowing and uniform. This solution was cannulated dropwise into300 mL of ice cold DI water. The pH of this solution was then adjustedto 4.5 with 350 mL of concentrated ammonium hydroxide (29% by weight,Product No. 5820, J. T. Baker, Phillipsburg, N.J.). The solution wasfiltered through a 10-μm polycarbonate filter (Isopore™ membrane filter,Catalog No. TCTP 04700, 47-mm diameter, Millipore, Billerica, Mass.)with constant mechanical stirring to prevent buckypaper from forming onthe filter surface. Filtration was run by continuously adding 100 mL ofDI water until the solution was clear.

Next, 670 mg of wet tubes were sonicated overnight in 1 liter of pH 8.5ammonium hydroxide at 3° C. The concentration (measured by a filterdisk) was 0.364 g/L. The solution was centrifuged at 22.5 krpm for 30minutes, and no significant residue fell out. The D/G ratio was measuredat 0.082 (633-nm laser; see FIG. 1). The metals content was found to be3.1% metals by thermogravimetric analysis (TGA; see FIG. 2). Forcomparison, the raw carbon nanotube material was 8.4% metals by TGA(FIG. 3). TGA was run on a wet sample (0.7449% solids). A decompositionpeak occurred at 294.66° C. T_(Onset) was 503.94° C. A number of peaksoccurred at 526.90° C., 539.44° C., and 554.32° C.

The solution was spray coated onto glass, and the sheet resistance ofthe film was measured with respect to % transmittance (% T; Table 1).The % T decreased as the number of coats increased. The low sheetresistance of the resulting films indicated non-covalentfunctionalization, because covalent functionalization introducesdefects, and defects decrease conductivity.

TABLE 1 % T SHEET RESISTANCE (Ohm/Sq) 96.6 1.88 × 10⁵ 94 3.72 × 10⁴ 92.51.39 × 10⁴ 90.1 5.32 × 10³ 87.9 5.37 × 10³ 86.5 2.53 × 10³

Example 2 Reaction of Carbon Nanotubes with 1-Pyrenebutyric Acid andGraphene

In this Example, 200 mg of CG200 carbon nanotubes, 200 mg of1-pyrenebutyric acid, and 200 mg of xGnP® graphene nanoplatelets(25-micron diameter, lot SU52909, XG Sciences, Lansing, Mich.) wereplaced in a 250-mL Schlenk flask and flushed with nitrogen. Then, 276grams of fuming sulfuric acid (20% free SO₃) were cannulated into theflask. The solution was stirred for 2 days. The solution was freeflowing and uniform. This solution was cannulated dropwise into 250 mLof ice cold DI water. Next, 300 mL of ammonium hydroxide (24% by weight,Product No. 09870, Sigma Aldrich, St. Louis, Mo.) were then addeddropwise. The resulting solution was then filtered through a 10-μmpolycarbonate filter with constant mechanical stirring to preventbuckypaper from forming on the filter surface. Filtration was run bycontinuously adding 100 mL of DI water until the solution was clear.

The recovered wet solid was sonicated into 250 mL of pH-18.8 ammoniumhydroxide and centrifuged at 22.5 krpm for 30 minutes. The resultingsolution had an Optical Density (OD) of 1.85 at 550 nm. Theconcentration was 0.089 g/L by filter disk. The D/G ratio was measuredat 0.119 (633-nm laser; see FIG. 4). For comparison, the raw CG200 tubeshad a D/G ratio of 0.016 (633-nm laser; see FIG. 5). The Raman spectraindicated that the tubes were not highly functionalized. Significantpeaks in the radial breathing mode were observed at 192.6 nm, 217.7 nm,and 254.0 nm (633-nm laser, FIG. 6). Raw CG200 tubes had significantradial breathing mode peaks at 215.5 nm, 241.5 nm, 254.3 nm, 335.7 nm,and 344.4 nm (633-nm laser, FIG. 7).

The solution was spray coated on glass, and the sheet resistance of thefilm was measured with respect to % transmittance (% T; see Table 2).

TABLE 2 % T SHEET RESISTANCE (Ohm/Sq) 97.3 1.40 × 10⁵ 92.6 1.59 × 10⁴85.6 2.79 × 10³ 84.9 2.61 × 10³ 76.1 1.23 × 10³ 72.7 9.52 × 10² 70.76.79 × 10²

Example 3 Reaction of Semi-Conducting Carbon Nanotubes with1-Pyrenebutyric Acid

In this procedure, 320 mg of SG65 carbon nanotubes (SouthWestNanoTechnologies, Norman, Okla.), and 170 mg of 1-pyrenebutyric acidwere placed in a 250-mL Schlenk flask and flushed with nitrogen. Then,328.2 grams of fuming sulfuric acid (20% free SO₃) were cannulated intothe flask. The solution was stirred overnight at room temperature. Thissolution was cannulated dropwise into 250 mL of ice cold DI water. Then,250 mL of 29% w/v ammonium hydroxide were added dropwise. This solutionwas diluted with 8 liters of deionized water in a 10-liter reactorvessel. The solution was filtered using cross-flow filtration and wasthen recovered in 8 liters of deionized water. Next, 3 mL of 29% w/vammonium hydroxide were added, and the solution was sonicated for 2hours. Again, the solution was filtered using cross-flow filtration andwas recovered in 8 liters of deionized water. This filtration wasrepeated one more time, the tubes were recovered in 4 liters ofdeionized water, and the pH was adjusted to 8.5 with 29% w/v ammoniumhydroxide.

This solution was finely dispersed. It was centrifuged twice for 30minutes at 22.5 krpm. The resulting solution had an OD of 0.60 at 550nm. The concentration was 0.030 g/L (as measured by filter disk). TheD/G ratio was measured at 0.226 (633-nm laser, see FIG. 8). Raw SG65tubes had a D/G ratio of 0.18 (633-nm laser, see FIG. 9). Significantpeaks in the radial breathing mode were at 216.7 nm, 255.0 nm, 281.8 nm,293.6 nm, 306.0 nm, 332.5 nm, and 417.3 nm (633-nm laser, see FIG. 10).Raw SG65 tubes had significant radial breathing mode peaks at 189.7 nm,213.4 nm, 252.5 nm, 279.6 nm, 303.8 nm, and 332.2 nm (633-nm laser,spectrum 9).

The solution was spray coated on glass, and the sheet resistance of thefilm was measured with respect to % transmittance (see Table 3). Thesheet resistance was quite low for a semi-conducting film.

TABLE 3 % T SHEET RESISTANCE (Ohm/Sq) 95.3 1.88 × 10⁵ 94.6 3.72 × 10⁴92.9 1.91 × 10⁴ 87.6 5.68 × 10³ 85.1 4.99 × 10³ 80.6 2.71 × 10³ 79 2.33× 10³

Example 4 Reaction of Carbon Nanotubes with 1-PyrenemethylamineHydrochloride

In this Example, 840 mg of CG200 carbon nanotubes and 1,230 mg of1-pyrenemethylamine hydrochloride (Product No. 401633, Sigma Aldrich,St. Louis, Mo.) were placed in a 3-liter, 3-neck round-bottom flask andflushed with nitrogen. Then, 1.35 liters of concentrated sulfuric acid(Product No. 435589, Sigma Aldrich, St. Louis, Mo.) were poured into theflask. This solution was cannulated dropwise into 3.0 liters of an icecold solution of 50% by volume ammonium hydroxide (29% by weight) in DIwater. The resulting solution was stirred overnight. After stirring, thesolution was diluted with 8 liters of DI water in a 10-liter reactorvessel. This solution was sonicated for 2 hours (Blackstone-NeyUltrasonics Model PROHT 1212 sonicator with Neptune Ultrasonicsgenerator Model N1500-C-XHSKA-120-480/12 with potentiometer set atmaximum, power having been measured at 40-45 W/in²) with the temperatureset at 3° C. The solution was filtered using cross-flow filtration withsonication and then recovered in 8 liters of deionized water twice.Next, 3 mL of 29% w/v ammonium hydroxide were added, and the solutionwas filtered using cross-flow filtration with sonication and recoveredin 4 liters of deionized water. Finally, 3 mL of 29% w/v ammoniumhydroxide were added, and the solution was sonicated for 2 hours.

The resulting solution was highly dispersed. It was centrifuged for 30minutes at 22.5 krpm. A 1:2 dilution of this solution in DI water had anOD of 1.42 at 550 nm. Its concentration was measured to be 0.132 g/L byfilter disk. After filtration, 3.4753 grams of wet product wererecovered. TGA of this sample determined that it was 18.67% solids,giving a total weight of 0.649 grams, or 81% yield. A smalldecomposition was observed at 272.62° C. The T_(Onset) was found to be500.48° C., and residue was 12.91%.

The D/G ratio was measured at 0.010 (633-nm laser, see FIG. 11). RawCG200 tubes had a D/G ratio of 0.016 (633-nm laser, see FIG. 5).Significant peaks in the radial breathing mode were at 197.3 nm, 220.4nm, 255.8 nm, 284.5 nm, and 336.8 nm (633-nm laser, see FIG. 12). RawCG200 had significant radial breathing mode peaks at 215.5 nm, 241.5 nm,254.3 nm, 335.7 nm and 344.4 nm (633-nm laser, see FIG. 7).

The solution was spray coated on glass, and the sheet resistance of thefilm was measured with respect to % transmittance.

TABLE 4 % T SHEET RESISTANCE (Ohm/Sq) 95.4 1.19 × 10⁴ 92.8 4.18 × 10³91.8 3.31 × 10³ 86.4 1.40 × 10³ 86.9 1.46 × 10³ 84.4 1.12 × 10³ 84 9.66× 10² 79.1 6.00 × 10² 73.7 4.50 × 10²

Example 5 Reaction of Carbon Nanotubes with 1-PyrenemethylamineHydrochloride

In this Example, 458 mg of XBC3350 carbon nanotubes (Continental CarbonNanotechnologies, Inc. 16850 Park Row, Houston, Tex., 77084) and 484 mgof 1-pyrenemethylamine HCl (Product No. 401633, Sigma Aldrich, St.Louis, Mo.) were placed in 150 mL of chlorosulfonic acid (Product No.571024, Sigma Aldrich, St. Louis, Mo.) under nitrogen. This was stirredfor 3 days. The solution was cannulated drop wise into 409 grams of iceand kept cold in an ice bath. Carbon nanotubes remaining in the flaskwere washed into the quenched solution with 20 mL of DI water. Using anaddition funnel, 290 mL of 29% w/v ammonium hydroxide (29% by weight,Product No. 5820, J. T. Baker, Phillipsburg, N.J.) were added drop wise.The solution was kept cold during this process with an ice bath. Thesolution was stirred overnight at room temperature. The solution wasthen cooled in ice and 250 mL of 29% w/v ammonium hydroxide were addeddropwise. The pH was measured to be 9.8 at the end of this process.

A portion of this carbon nanotube slurry was then filtered for two daysthrough a 10-μm polycarbonate filter. The resulting carbon nanotube cakehad partially dried and looked and behaved like clay. The remainder ofthe slurry was added to the filter washing the flask with 100 mL DIwater into the filter. The carbon nanotube clay was removed from thefilter and 250 mL of pH 10.3 ammonium hydroxide in DI water was added insmall portions while vigorously stirring. This addition began with 5 mLportions each time, mixing the clay with the ammonium hydroxide until itwas uniform. Eventually, it became a gel. When the solution wassufficiently fluid, 50 mL portions were added with vigorous stirring.Once all 250 mL of ammonium hydroxide were added, the mixture wasfiltered through a 10-μm polycarbonate filter.

This CNT clay was sonicated into 600 mL of pH 10.3 ammonium hydroxide inDI water in two, 300-mL portions. Sonication conditions were 45 minutesat 90% power with a 1-inch probe with the booster attachment. Duringthis process, the solution was in a cooling bath set at 5° C. Thissolution was centrifuged at 23.5 krpm for 30 minutes. The final solutionwas pipetted off of the top of the centrifuge tube.

The concentration of this solution was measured to be 1.088 g/L based ona filter disk. The solution was very well dispersed, easily passingthrough a 10-μm polycarbonate filter without leaving any residue. Thesolution was diluted 10× and spray coated on glass to measure sheetresistance vs % transparency (shown in Table 5). The D/G ratio wasmeasured at 0.109 (633-nm laser, see FIG. 13). Raw XBC3350 tubes had aD/G ratio of 0.057 (633-nm laser, see FIG. 13). Significant peaks in theradial breathing mode were at 164.6 nm, 221.8 nm, 255.1 nm, and 338.2 nm(633-nm laser, see FIG. 14). Raw XBC3350 tubes had significant radialbreathing mode peaks at 154.5 nm, 166.0 nm, 218.6 nm, 256.5 nm, and337.2 nm (633-nm laser, see FIG. 14).

TABLE 5 % T SHEET RESISTANCE (Ohm/Sq) 94.4 1.51 × 10³ 88.3 5.35 × 10²84.5 3.15 × 10² 79.6 2.29 × 10² 77.1 2.06 × 10²

Example 6 Reaction of Carbon Nanotubes with Rubrene

In this procedure, 220 mg of carbon nanotubes and 230 mg of rubrene(Product No. R2206, Sigma Aldrich, St. Louis, Mo.) were stirred in 100mL of fuming sulfuric acid (20% free SO₃, Product No. 435597, SigmaAldrich, St. Louis, Mo.) under nitrogen for 2 days. This was thenquenched dropwise into 250 mL of ice-cooled, DI water. Next, 250 mL ofammonium hydroxide (24% by weight, Product No. 09870, Sigma Aldrich, St.Louis, Mo.) were added dropwise over 1 hour. This solution was pouredinto a 10-liter reactor with 4 liters of DI water and sonicated for 17hours. The pH at this point was measured to be 1.98. The solution wasfiltered through a cross flow filtration and then recovered in 2 litersof DI water. The pH was adjusted with 6 mL of ammonium hydroxide (24%w/v) sonicated for 2 hours, and then filtered through the cross flowfiltration. Again, the solution was recovered in 2 liters of DI water,basified with 6 mL of ammonium hydroxide (24% w/v), and sonicated fortwo hours. The solution was then filtered again, recovered in 2 litersof DI water, and basified with 6 mL of ammonium hydroxide (24% w/v).This solution was sonicated overnight. Finally, 250 mL of this solutionwere removed and centrifuged at 23.5 krpm for 30 minutes.

The OD of this solution was measured to be 1.34 at 550 nm. The solutionwas spray coated on glass, and its sheet resistance was measuredrelative to transparency (Table 6).

TABLE 6 % T SHEET RESISTANCE (Ohm/Sq) 91.4 4.47 × 10⁴ 84.9 4.29 × 10³79.8 2.76 × 10³ 74.9 1.69 × 10³ 68.8 1.43 × 10³

The coatings from the initial solution were blotchy, which indicatedmore washing would be necessary to improve conductivity. The solutionremaining in the reactor was filtered using cross flow filtration,recovered in 2 liters of DI water, basified with 3 mL of 29% w/vammonium hydroxide, and then sonicated for 24 hours. Next, 250 mL ofthis solution were removed from the reactor and centrifuged at 23.5 krpmfor 30 minutes.

The OD of the second solution was measured to be 1.21 at 550 nm. Theconcentration was measured to be 0.0612 g/L based (determined by afilter disk). This solution was spray coated, and its sheet resistancewas measured relative to transparency (see Table 7). The coating wasuniform. The D/G ratio was measured to be 0.138 (633-nm laser, see FIG.15). Raw CG200 tubes had a D/G ratio of 0.075 (633-nm laser, see FIG.15). Significant peaks in the radial breathing mode were observed at192.4 nm, 216.0 nm, 254.9 nm, 282.5 nm, and 334.2 nm (633-nm laser, seeFIG. 16). Raw CG200 tubes had significant radial breathing mode peaks at193.5 nm, 218.6 nm, 257.1 nm, 283.7 nm, 337.5 nm and 346.1 nm (633-nmlaser, see FIG. 16).

TABLE 7 % T SHEET RESISTANCE (Ohm/Sq) 94.3 3.67 × 10⁴ 89.2 5.62 × 10³83.5 3.26 × 10³ 79.6 2.04 × 10³ 75.4 1.56 × 10³

Example 7 Reaction of Carbon Nanotubes with Fuming Sulfuric Acid (NoAromatic Hydrocarbon)

In this Example, 219 mg of CG200 carbon nanotubes were placed in 182.8grams of fuming sulfuric acid (20% free SO₃, Product No. 435597, SigmaAldrich, St. Louis, Mo.) under nitrogen and stirred overnight. Thesolution was cannulated into 250 mL of ice cold DI water. This solutionwas then neutralized with 250 mL of 29% w/v ammonium hydroxide (29% byweight, Product No. 5820, J. T. Baker, Phillipsburg, N.J.). This mixtureor dispersion was then filtered through a 10-μm polycarbonate filter(Isopore™ membrane filter, Catalog No. TCTP 04700, 47-mm diameter,Millipore, Billerica, Mass.) with stirring to prevent buckypaper fromforming on the filter surface. The resulting tubes were stirred into 500mL of DI water and filtered three times. The tubes were then placed in500 mL of pH 10.5 ammonium hydroxide and sonicated overnight in the bathsonicator. The solution was highly bundled with no dispersion,indicating that the reaction of CNTs with fuming sulfuric acid wasinsufficient to functionalize the tubes.

Example 8 Reaction of CG200 Carbon Nanotubes with 1-PyrenemethylamineHydrochloride (No Acid)

In this procedure, 202 mg of CG200 carbon nanotubes, 210 mg of1-pyrenemethylamine hydrochloride, and 1 liter of pH 10.2 ammoniumhydroxide in DI water were combined in a 10-liter reactor. The solutionwas sonicated in the bath sonicator for 12 hours, and checked fordispersion. The mixture was not dispersed. The mixture was sonicated foran additional 17 hours and checked for dispersion. The mixture was stillnot dispersed. A liter of DI water was added to reduce theconcentration. The mixture was then sonicated for another 24 hours andchecked for dispersion. Again, the mixture was still not dispersed,indicating that the 1-pyrenemethylamine HCl was not sufficient tosolubilize or disperse the CNTs in the absence of the strong acid underthe above conditions.

Example 9 Reaction of Carbon Nanotubes with 1,3,6,8-PyrenetetrasulfonicAcid Tetrasodium Salt Hydrate (No Acid)

In this Example, 367 mg of CG200 carbon nanotubes, 267 mg of1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt hydrate (Product No.82658, Sigma Aldrich, St. Louis, Mo.), and 250 mL of pH 10.0 ammoniumhydroxide were sonicated for 15 hours then allowed to sit overnight.This solution was centrifuged at 22.5 krpm for 30 minutes, and thedispersed liquid was collected. The OD of the resulting solution wasmeasured to be 0.53 at 550 nm. This solution was spray coated on glass,but none of the slides were conductive. This demonstrated thatsulfonated pyrene compounds can solubilize carbon nanotubes, but theresulting solutions are very dilute. Additionally, the solutions thatresult from this process do not produce conductive coatings when spraycoated.

Example 10 Reaction of 1-Pyrenemethylamine Hydrochloride with FumingSulfuric Acid to Characterize Reaction Products

In this Example, 1.958 grams of 1-pyrenemethylaminehydrochloride(Product No. 401633, Sigma Aldrich, St. Louis, Mo.) were placed in 75 mLof fuming sulfuric acid (20% free SO₃, Product No. 435597, SigmaAldrich, St. Louis, Mo.) under nitrogen and stirred at room temperaturefor 24 hours. This mixture was cannulated into 75 mL of DI water chilledin an ice bath. Then, 75 mL of 24% w/v ammonium hydroxide (Product No.09870, Sigma Aldrich, St. Louis, Mo.) were added drop wise, whilecontinuing to cool. The pH was measured to be 0.3. Ammonium hydroxidewas then slowly added until the pH was 7.7. The temperature was keptbelow 25° C. during the entire process.

After 5 days, a yellow precipitate had formed. The solution was shaken,and a small aliquot was taken to test solubility characteristics. Theprecipitate quickly dissolved when 0.5 mL of 24% w/v ammonium hydroxidewas added. The precipitate was filtered through a 10-μm polycarbonatefilter, and the solid was collected. This solid dissolved in 200 mL ofDI water easily. The solution was placed in a 400-mL beaker and stirredunder a stream of air with the hotplate set at 107° C. This liquid wasconcentrated down to about 3 mL before it started to precipitate out,followed by filtering to yield a silvery wet solid. This was dissolvedin 3 mL of DI water and placed in a scintillation vial. The solution wasagain evaporated until it precipitated out. The solid was collected anddried under vacuum.

Next, 10 mg of the dried solid was placed in 1.0 mL of deuterated waterin a 0.7 mL NMR tube. The spectrum was taken on a 400 MHZ INOVA 400Varian NMR. Significant peaks were observed at 8.976 ppm (s), 8.748 (d,JH=6.0 Hz), 8.692 (d, JH=6.0 Hz), 8.522 (d, JH=6.3 Hz), 8.245 (s), 7.707(d, JH=5.8 Hz), 4.624 (s, isotopic satellites JD=26.5 Hz), and 4.401(s). This spectrum is shown in FIG. 17.

A mass spectrum was obtained by dissolving the sample in 50:50methanol:water and injecting the filtered sample directly into the ESImass spectrometer. FIG. 18 shows the mass spectrum of the reactionproduct. Table 8 shows the isotropic ion abundances for the molecularmass region.

TABLE 8 m/z REL. INTENSITY 455 78 456 17.0 457 13.0 472 100 473 22.1474. 16.5

The molecular mass of the reaction product was 471 Da. The isotropicabundances indicated that the molecular formula was C₁₇H₁₃NO₉S₃. Basedon the ¹H NMR and the mass spectrum, the reaction product was1-pyrenemethylamine-3,6,8-trisulfonic acid (see FIG. 19).

Example 11 Attempted Solubilization of Carbon Nanotubes with1-Pyrenemethylamine-3,6,8-Trisulfonic Acid

A purified sample (1.58 grams) of 1-pyrenemethylamine-3,6,8-trisulfonicacid synthesized as described in Example 10 was placed in a stainlesssteel beaker with 213 mg XBC 3350 carbon nanotubes (Continental CarbonNanotechnologies, Inc., Houston, Tex.) in 250 mL of pH 10.3 ammoniumhydroxide. The slurry was sonicated with a 1-inch probe sonicator set at90% power for 45 minutes in a chilled bath, and the solution was allowedto cool. The slurry was sonicated for an additional 1 hour, and thencentrifuged at 23.5 krpm for 30 minutes. The measured optical density ofthe resulting solution at 550 nm was 2.25. The concentration (measuredby filter disk) was 0.0736 g/L. Both the OD and the concentration of thesolution were an order of magnitude lower than that obtained by theinventive process of Example 5.

We claim:
 1. A method of preparing a carbon nanotube dispersion, saidmethod comprising: providing a mixture of carbon nanotubes, a compoundcomprising at least one polyaromatic moiety, and an acid; noncovalentlybonding said carbon nanotubes with said compound comprising at least onepolyaromatic moiety; and reacting said acid with said at least onepolyaromatic moiety.
 2. The method of claim 1, wherein before saidproviding, at least some of said carbon nanotubes are bundled orclustered, and during said providing, said acid separates at least someof said bundled or clustered carbon nanotubes.
 3. The method of claim 1,wherein said noncovalently bonding and said reacting occur substantiallysimultaneously.
 4. The method of claim 1, wherein said providingcomprises first mixing said carbon nanotubes with said acid, and thenadding said compound comprising at least one polyaromatic moiety.
 5. Themethod of claim 4, wherein at least some of said carbon nanotubes arebundled or clustered, and during said mixing, said acid separates atleast some of said bundled or clustered carbon nanotubes.
 6. The methodof claim 1, wherein said providing comprises first mixing said compoundcomprising at least one polyaromatic moiety with said acid, and thenadding said carbon nanotubes.
 7. The method of claim 1, wherein saidcarbon nanotubes are selected from the group consisting ofsingle-walled, double-walled, and multi-walled carbon nanotubes.
 8. Themethod of claim 1, wherein said compound comprising at least onearomatic moiety is selected from the group consisting of substituted andunsubstituted compounds selected from the group consisting ofnaphthalene, anthracene, phenanthracene, pyrene, tetracene, tetraphene,chrysene, triphenylene, pentacene, pentaphene, perylene, benzo[a]pyrene,coronene, anthanthrene, corannulene, ovalene, graphene, fullerene,cycloparaphenylene, polyparaphenylene, cyclophene, and compoundscontaining moieties of the foregoing.
 9. The method of claim 1, whereinsaid acid has a pKa of less than about −1.
 10. The method of claim 1,wherein said acid is selected from the group consisting of sulfuricacid, chlorosulfonic acid, triflic acid, p-toluenesulfonic acid, andmixtures thereof.
 11. The method of claim 1, wherein said carbonnanotubes and compounds comprising at least one polyaromatic moiety areprovided in quantities such that the molar ratio of carbon nanotubes topolyaromatic moieties is from about 25:75 to about 75:25.
 12. The methodof claim 1, wherein said acid is present at levels of from about 90% toabout 99.99% by weight, based upon the total weight of the dispersiontaken as 100% by weight.
 13. The method of claim 1, wherein the carbonnanotube dispersion resulting from said noncovalently bonding andreacting has a carbon nanotube concentration of at least about 0.05% byweight, based upon the total weight of the dispersion taken as 100% byweight.
 14. The method of claim 1, wherein said carbon nanotubedispersion resulting from said noncovalently bonding and reacting can beformed into a film having a sheet resistance of less than about 7,000Ω/sq.